A stent for placement (indwelling) in living bodies is generally a tubular medical device which, for treatment of various diseases generated by stenosis or occlusion of a blood vessel or other living body lumen, is placed (put indwelling) in the stenosed or occluded lesion so as to dilate the lesion and secure the lumen at the lesion.
The following description of the stent is set forth describing one example of the use of the stent, namely in a blood vessel. This example should not be construed as a limitation.
The stent is a body which, for insertion from the outside into the inside of a living body, is relatively small in diameter at the time of insertion, and is expanded at the target stenosed or occluded lesion so as to be enlarged in diameter and to maintain the lumen at the lesion.
In general, stents are cylindrical bodies obtained by processing metallic wires or metallic pipes. A stent is mounted to a catheter or the like in a radially reduced state, is inserted into a living body, and is expanded at a target lesion by some method, to be fixed in close contact with the inner wall of the lumen at the lesion, thereby maintaining the lumen shape. Stents are classified by function and placement method as self-expandable stents and balloon-expandable stents. A balloon-expandable stent is a stent which itself does not have a self-expanding function or ability. The balloon-expandable stent is used together with a balloon in which the stent is mounted on the balloon and is inserted into a target lesion, and thereafter the balloon is dilated to expand (plastically deform) the stent by the expanding force of the balloon, whereby the stent is fixed in close contact with the inner surface of the target lumen. This type of stent requires the stent-expanding operation described above. On the other hand, a self-expandable stent is a stent which itself has a self-expanding function. The use of this self-expandable stent involves inserting the stent in a radially contracted state into a living body, and releasing the stent from the contracted state at a target part so as to allow the stent to return to its original expanded state, thereby being fixed in close contact with the inner wall of the lumen at the lesion and maintaining the lumen shape.
The purpose of the placement of a stent at present is to return a blood vessel stenosed for some reason to its original open (patent) state. In most cases, the stents are mainly for preventing or reducing the risk or extent of restenosis which might occur after such a procedure as PTCA. In recent years, for suppressing the probability of restenosis more securely, drug-eluting stents with such a drug as immunosuppressor or carcinostatic loaded thereon are used, and the effect thereof is generally known.
On the other hand, with regard to treatment of acute coronary syndromes represented by acute myocardial infarction and unstable angina or treatment of unstable plaque which is said to be pre-stage of the syndromes, an effective method has not yet been established. As to the treatment of acute coronary syndromes, the existing stents and drug-eluting stents are in contraindication in the status quo. The reason for this status lies in that the placement of a stent in a blood vessel including a large amount of thrombus cannot be free from the risk of stent malapposition and long-term thrombosis.
In relation to the unstable plaque, a thought called plaque sealing in which the plaque is stabilized by giving a stimulus to the relevant surface by balloon dilation or the like has been proposed by Meler et al (Heart 2004; 90: 1395-1398: Plaque sealing by coronary angioplasty). In recent years, it has been reported that plaque sealing is carried out by use of a self-expandable stent having a comparatively weak expanding force, in place of a balloon.
Most of the self-expandable stents are used in peripheral regions such as inferior-limb blood vessels and carotid arteries; in the coronary region, on the other hand, only the Radius stents made by Boston Scientific have been marketed in the past. The Radius stents have a form as shown in JP-T Hei 11-505441. In this type of stent, due to the properties of the stent, the positioning of the stent at the time of placement is difficult as compared with balloon-expandable stents. In fact, the occurrence of the so-called jumping phenomenon has been reported in which the stent is moved excessively from the sheath when placed (put indwelling).
The following has also been reported in academic meetings or the like. When conventional balloon-expandable stents and self-expandable stents having strong expanding forces are used in the plaque sealing, the stent placing operation itself has the risk of rupturing the plaque. Once the plaque is ruptured, there would be the fear of a risk of peripheral occlusion and/or an increase of inflammatory reactions at the relevant part. Therefore, such conventional stents are unsuited to plaque sealing.
Another stent has been proposed as described in Japanese Patent Laid-open No. 2003-93519. The stent disclosed in this document includes a plurality of wave-shaped struts extending in the axial direction of the stent from one end side to the other end side of the stent and arranged in the circumferential direction of the stent, and a plurality of connecting struts which each interconnect a pair of the wave-shaped struts adjacent to each other and which extend in the axial direction over a predetermined length, wherein each wave-shaped strut has an end portion joined to an end portion of the wave-shaped strut adjacent thereto. Since the stent is composed of the plurality of wave-shaped struts extending in the axial direction of the stent, the stent is flexible and may have possible application to plaque sealing.
In the stent described in Japanese Patent Laid-open No. 2003-93519, the apexes of the wave-shaped struts are located at the same positions with respect to the axial direction of the stent. In other words, the apexes of the plurality of wave-shaped struts are located in an annular fashion.
Studies conducted by the present inventors have led to the discovery that the stent described in Japanese Patent Laid-open No. 2003-93519 exhibits flexibility due to the use of the plurality of wave-shaped struts extending in the axial direction of the stent, but the stent as a whole lacks a smoothly bendable property. It is believed that this may be attributable to the fact that, in the above-described form of the stent, there is a large difference in physical properties between the annular regions where the apexes are present and the annular regions where the apexes are absent. It was found that, since the annular regions where the apexes are present are curved extremely flexibly as compared with the annular regions where the apexes are absent, the stent takes such a form that it is bent at its joint portions (the annular regions where the apexes are present).